keeping amps cool?

Yes, keeping cool extends life, electronics and humans. Try some of the laptop computer coolers, very inexpensive, quite and effective.

I would only aim a fan at the cooling fins, unless I was certain the amp did not have a built in self bias scheme.
The Parasound HCA-3500, for instance, heats up appreciably until it reaches a set temperature.
We took the top cover off my friend's HCA-3500, to see if it would run cooler. It biased up even higher, with the cover off..

Actually you're better off using the fan to draw heat from the amp instead of blowing air directly on it.

You will skew the setting even on a fixed bias amp.

"Actually you're better off using the fan to draw heat from the amp instead of blowing air directly on it."

yes, I use a PQ exhaust fan...
http://www.hometheatercooling.com/purchase.html

Drawing the heat is easy. anyone else concur it's better than blowing filtered ionized air -gently--at the fins?
I believe in efficiency and know blowing is more efficient than drawing away.

Buy a Class D amp if you're concerned with efficiency.

In the past I had an amp stand constructed that incorporated 2 whisper fans that were used to draw the heat away from the amp thereby “enhancing” the amp’s nature convection cooling properties.

The fan’s had a variable power supply to control fan speed that could not be heard from the listening position.

Push me pull me, what's the diff? I use an O 2 Cool outboard fan directed at the tubes of my Bada Class A headphone amp that normally runs quite warm. The tubes stay cool as a cucumber. O 2 Cool fans run very quietly, available on Amazon.

How does that change the sound, Geoff?

Some amp's come with a thermal controlled fan, it only works when needed.

That's right. And some come with one that operates constantly. Some don't come with one at all. Hmmmm.

Cool is not to change the sound. It's o extend the operating life of the caps and perhaps the whole amp. Heat kills. Specifically I enjoy a Spectral DMA 180 and am happy with the sound-so would like to keep it a long time; without having to ship it across the US/Can border. Always some danger in shipping.

Unless it operates at the temperature it was designed to, it will definitely change the sound. Capacitors typically never get anywhere near their highest allowable temp. Those parts that are vulnerable to thermal breakdown are the ones on a heat sink. If your amp is functional after 20 years, it still needs to be rebuilt.

I use a 24v muffin fan run at 12v and set it on my Mac c2200 tube preamp wired to a door switch that turns on when in use. Works well sucking heat from within chassis.

Why does it need rebuilding if it still seems to function fine?

That's just it. It 'seems' to function. Although it may sound okay, it has by then fallen probably far below spec. If you owned it since new, you probably didn't notice it's deterioration over the years. If it's a great amp worthy of a good overhaul, you're in for a treat. It will likely sound better than new due to current technology parts and maybe a mod or two by a respected tech or engineer.

Ptss,
I would agree with Csontos, and suggest to seriously consider mods such as replacing and upgrading with better quality capacitors, internal wires , resistors, diodes and rectifiers , binding posts ,etc. Actually they do not cost so much ( compared to buying a new amp) but improvement in performance will be outstanding. I did it on my Cary Audio (AES SuperAmp AE-25 ) and thoroughly satisfied , even people at Cary were surprised at the good result.
Reg cooling : I had on the same amp custom installed a 3" cooling fan at the bottom to take air from underneath and throw into the body of amp. I had even a 2 speed regulator installed which make fan to run at higher speed ( more noise /dB ) while amp on stand bye , but run at half speed while amp in operate mode ( very low noise/dB ) . It works great !

I would leave upgrade work to Spectral.
Who would you suggest?

Any tech with more knowledge than just trouble shooting. Someone who specializes in this kind of work with lots and only good feedback. You have to do your home work, seriously.

My homework says Spectral will do just fine. Did you have anyone in mind?

I had four Acoustat TNT amps rebuilt and modded by Roy Esposito in Florida recently. He was a principal engineer in that company and instrumental in the amp's design. He now runs a shop dedicated to preserving the Acoustat line. He added balanced inputs and mono capability along with the full rebuild/upgrade. The amps are very impressive. I've had them for about a year now and they just seem to get better and better. They never cease to surprise me. I cannot find fault with them. They have everything in spades. However, he is not cheap. Cost of each amp was about 500 and the work about 900 each. To use his words, "This amp will run circles around amps costing ten to fifteen thousand". He wasn't lying. Imo, he was being conservative. I believe he's willing to take on other brands as well.

In Toronto there's a guy who used to work at Brack's, a high end audio shop, who now runs his own show. Very highly sought after. His name eludes me but I could round it up.